Apparatus for transmitting bearing information

Abstract

An electronic navigation beacon, particularly for coastal navigation, in which an unmodulated carrier f c is transmitted from the first antenna of a pair of antennas, and a periodically modulated sideband f c ±f a is transmitted from both antennas, where f a is in the audio frequency domain. A cyclical RF phase variation is imposed on the sideband signal transmitted from the second antenna. The effect in space is that of a rotating radial having an audio frequency null, the periodic modulation providing the listener at a remote point with timing information enabling determination bearing as a function of the time of passage of the radial through the remote point relative to the start of rotation of the radial.

We claim: 
     
       1. Apparatus for transmitting bearing information, comprising: first and second fixed antennas spaced a first predetermined distance apart;   first means for transmitting an unmodulated VHF carrier signal of frequency f c  from a first one of said antennas;   second means for transmitting an unmodulated VHF signal of a single frequency selected from f c  +f a  and f c  -f a , where f a  is an audio frequency;   third means for cyclically varying the RF phase of one of said first and second means transmitted signals relative to the other to produce a rotating signal null at a distance from said apparatus;   fourth means for periodically modulating said second means selected signal to convey timing information relating to the rotation of said signal null, the duration of each of said third means cyclical RF phase variations being a multiple of the period of said fourth means modulation;   fifth means for inhibiting transmission of signals from at least one of said antennas which is subject to said third means cyclical RF phase variation during intervals between successive ones of said third means cycles;   sixth means comprising a third antenna spaced a second predetermined distance from said first antenna, and means for feeding said third antenna with the output of said fourth means;   and seventh means for cyclically varying the RF phase of the signal transmitted by said third antenna synchronously with said phase variation of the same signal from said second antenna but offset in phase by a predetermined amount such that the two antenna pairs formed by said first and second antennas and by said first and third antennas each produce coincident rotating radials on one side of their respective baselines and angularly separated rotating radials on the opposite side of said baselines.